2 Answers
2

Ever wanted to get rid of Outlook ?
DavMail is a POP/IMAP/SMTP/Caldav/LDAP
exchange gateway allowing users to use
any mail/calendar client (e.g.
Thunderbird with Lightning or Apple
iCal) with an Exchange server, even
from the internet or behind a firewall
through Outlook Web Access. DavMail
now includes an LDAP gateway to
Exchange global address book to allow
recipient address completion in mail
compoze window and full calendar
support with attendees free/busy
display.

There are some others such as KMail and Mailody (both for KDE), which advertise Exchange support, although on numerous sites I've found that either IMAP must be enabled or certain features such as the Exchange Calendar would not work -- which is vital in the workplace if you need to receive notices of meetings and such.

A similar question was brought up on Slashdot a little while ago, and judging by the comments it appears that a virtual machine of Windows running Outlook is the way to go if you want to ensure compatibility.